1. Technical Field
The present invention relates to a method of treating an azide-containing waste and, more particularly, to a method of treating a waste which contains an ignitable mixture of an alkali metal azide and an oxidant.
2. Description of the Prior Art
Alkali metal azides are used in the manufacture of gas generating material for inflatable vehicle occupant restraints, such as air bags. The alkali metal azides are mixed with other ingredients, including reactants for the alkali metal azides. Common reactants for the alkali metal azides are a metal oxide, such as iron oxide or copper oxide, and a water-soluble oxidant, such as sodium nitrate The mixture of ingredients is formed into a gas generating material. When ignited, the gas generating material produces nitrogen gas.
The gas generating material may be formed into gas generating grains as shown in U.S. Pat. No. 4,817,828. The grains may also contain ingredients, such as carbon fibers, which impart mechanical strength to the grains, and bentonite, which functions as an extrusion aid in forming the grains.
The gas generating grains are susceptible to fracture from handling, shipping and storage. When fractured, the grains are unsuitable for use in a vehicle occupant restraint. The fractured grains thus become solid waste.
One method of manufacturing the gas generating grains involves wet blending the ingredients. When the equipment used to wet blend the ingredients is cleaned, a liquid waste containing the ingredients is generated.
Alkali metal azides are expensive. Therefore, attempts have been made to recover a substantial amount of the azides from the solid and/or liquid waste. This involves the use of separation equipment to separate the azides from the other ingredients of the waste. However, during this recovery, an effluent is obtained which will contain some residual azide. The effluent will also contain other ingredients, such as a metal oxide. Thus, the effluent may be capable of combustion and is unsuitable for landfill or sewer disposal.
The publication "Energetic Materials", Plenum Press, New York (1977), Volume 2, Chapter 3, entitled "Handling, Storability and Destruction of Azides", discloses several procedures for the disposal of waste containing an alkali metal azide. One approach involves burning the waste in a large incinerator or open pit. The azide decomposes leaving behind a residue of oxides of the alkali metal. These oxides can react with water to form alkali metal hydroxide. The alkali metal hydroxide is in concentrated form and must be neutralized with an acid and then removed from insoluble solids in the residue by filtering, prior to disposal, all adding to the cost of disposal. Also, incineration or open pit burning can generate hazardous gaseous by-products, such as active oxides of the sodium metal and oxides of nitrogen.
The publication also discloses disposal of an azide-containing waste by neutralizing tho azide with an acidified solution of sodium nitrite (NaNO.sub.2) and water. The problem with this process is that hydrazoic acid fumes (HN.sub.3) can be generated during the neutralization reaction. Hydrazoic acid fumes are explosive and highly toxic in very small concentrations (for instance 100 parts per billion in air). In addition, other toxic fumes, such as nitrous oxide (N.sub.2 O), can be generated in the neutralization reaction.